The ability of a fuel injector to respond to an input signal's command to open is a significant factor in the fuel injector's ability to deliver a precise injection of fuel to a combustion chamber. Parameters that define the fuel injector's magnetic circuit (e.g., the stator, the armature, and the working gap between the stator and the armature) are of particular importance since it is this magnetic circuit that conducts the magnetic flux that exerts the magnetic force which acts on the armature. The rate at which the magnetic flux builds determines the rate at which force acting on the armature builds. The faster the force builds, the faster the fuel injector opens.
While it is recognized that magnetic flux cannot be built instantaneously, it has been conventional practice to use various fuel injector driver circuits that seek to maximize the building of electric current in the solenoid's coil in the expectation that this will necessarily also maximize the rate at which magnetic flux is built in the magnetic circuit, and as a consequence also minimize the fuel injector's opening time.
It has now been discovered that the transient building of magnetic flux does not occur uniformly over the transverse cross sectional area of the magnetically conductive material (i.e. the stator and armature) in the valve's magnetic circuit. Rather, flux must build first in the magnetically conductive material's "skin" before it can build in the interior of the material's cross section. This phenomenon is a physical characteristic of the magnetic circuit material and is in the nature of a time constant (albeit a small one) that delays the propagation of flux into the interior of the cross section. For convenience it will be referred to herein as the flux propagation delay characteristic. Consequently, for a given magnetic circuit structure, the building of flux at any given point within a transverse cross section of the structure in response to the building of current in the coil, is a transient phenomenon that is a function of the input current to the coil as a function of time and the particular location of that point within the cross section. The flux propagation delay characteristic is an inherent constraint on the ability of a magnetic circuit to build flux, irrespective of the ability of a driver circuit to build electric current in the solenoid's coil, so that minimizing the coil current build time is not necessarily conclusive of maximizing the building of magnetic flux during such a transient. Magnetic saturation too is an inherent physical characteristic of the magnetic material in the magnetic circuit that comes into play.
Stating the foregoing in a different way, it may be said that certain rates of current build during the transient building of magnetic force which, in the absence of the flux propagation delay characteristic, would be effective to build a uniform flux density over the transverse cross sectional area of the magnetically conductive material within a certain time, will instead within a like period of time when the flux propagation delay characteristic is taken into account, result in a magnetic flux pattern over a given transverse cross sectional area of the magnetically conductive material that is non-uniform; and if the coil is driven sufficiently hard during the transient, the pattern will, on account of magnetic saturation, consist of a magnetically saturated skin and a flux-poor interior wherein the total magnetic flux that is less than that which would be created in the absence of the flux propagation delay characteristic.
Force that builds as a transient during the time that the coil current is building and domains of the magnetically conductive material are becoming magnetized is a significant contribution toward opening the fuel injector. While a final steady state force (short of saturation) is a function of the cross-sectional area of the magnetically conductive material, the transient force has been found to be a function of the length of the magnetically conductive material's skin, as measured around the perimeter of its transverse cross-sectional area. While there is no precise definition for the skin, it is typically quite thin, for example only a few microns. Since the transverse cross sectional area of this "skin" is small, it is apt to saturate before the flux can propagate more interiorly of the cross section. Thus, full advantage of the total cross-sectional area of the magnetically conductive material cannot be taken during this transient condition, and hence the building of the transient force is constrained.
Where a fuel injector must comply with a specified opening force requirement, and certain dimensional constraints are also imposed on the size of the fuel injector, it may not always be possible to realize a solution with known technology. Accordingly, it is desirable to improve the probability of obtaining a solution, and it is toward this objective that the present invention is directed. Principles of the present invention endow a fuel injector with the ability to comply with a specified opening force requirement within an equal or smaller package size than heretofore possible with a solenoid-operated device. Moreover, principles can be incorporated through the use of conventional manufacturing procedures.
Another effect that is detrimental to the building of magnetic force is the phenomenon of eddy currents. Changing current in the solenoid's coil creates such currents in the magnetically conductive material and slows the opening of the fuel injector. Accordingly, it also would be beneficial if the solution that is afforded by the present invention were to also attenuate such eddy currents, and that can in fact be accomplished in the implementation of the invention.
Briefly, a presently preferred embodiment of the present invention is disclosed herein as a fuel injector valve having a novel magnetic circuit.
The magnetic circuit comprises a stator, an armature, and a working gap. Generally speaking, the invention comprises means for increasing the amount of magnetic material "skin" without a corresponding increase in package size. The increase in the amount of such skin is accomplished by inclusion of sets of slots in the magnetic material. The magnetic material also includes means for altering the circulation path for the eddy currents in a manner that is intended to attenuate their interference with building transient magnetic force.
In the disclosed preferred embodiment, the invention includes a stator having inner and outer cylindrical pole members extending from a circular annular end wall and forming a tubular space into which is disposed an electrically actuated solenoid coil for generating a magnetic field operative to displace the armature and open the fuel injector. The magnetic circuit of the preferred embodiment thus includes two parallel annular working gaps disposed between the armature and the free ends of the inner and outer pole members. Means for increasing the amount of stator skin without increasing its package size comprises slots running along the pole members, although broad principles of the invention contemplate that slots may be disposed along any portion of the magnetic circuit that conducts the flux that passes across the magnetic gap.
Other objects, advantages, and capabilities of the present invention will become more apparent as the description proceeds.